Access to an internal calibration reference system during flight is an important requirement for contemporary remote
sensing missions. L-3 Communications SSG-Tinsley has designed, built, and tested a novel internal calibration source
based on ribbon sources. Via a flip-in mirror, the source assembly couples light through the field stop of an off-axis reimaging
telescope to provide a reliable test of the following optics and electronics. Non-imaging illumination design
principles assure uniform illumination of the sensor focal plane at levels adjustable over a very wide dynamic range.
The source assembly can be packaged into a compact, lightweight, and efficient flight unit for convenient installation at
an accessible location on the telescope. While the prototype source was specifically designed to match the GIFTS
telescope as a representative example of an off-axis re-imaging telescope, the design principles are flexible to allow
optimization for any comparable telescope system.
Contemporary and emerging sensor systems typically require in-flight calibration reference sources. These are required to satisfy increasingly stringent specifications for stability, repeatability, dynamic range, absolute irradiance accuracy, and irradiance distribution uniformity, while meeting stray light, weight, and power constraints. While SSG has successfully designed and flight-qualified an internal calibration source for a telescope in a Schmidt configuration, future remote sensing programs are more likely to require telescopes in a 3-mirror off-axis re-imaging configuration. A major advantage to developing an internal calibration reference source for a re-imaging telescope is the availability of an intermediate field stop where the illumination from the calibration source can be inserted into the optical train. SSG's internal source design offers important advantages over existing approaches using in-flight blackbodies, including reduced volume, weight, and power requirements and the ability to generate multiple irradiance levels over a short period of time. The GIFTS (Geosynchronous Imaging Fourier Transform Spectrometer) telescope has been used as a representative platform to demonstrate this new internal calibration source, as it is representative of a design that may be used for future programs including the HES (Hyperspectral Environmental Suite) telescopes.
An Internal Calibration Unit (ICU) has been designed, built, and tested to meet a stringent set of quantitative performance requirements for an infrared sensor in a Schmidt telescope configuration. The design concept is readily applicable to re-imaging telescopes. The design optimizes optics, source design, and power supply electronics for irradiance accuracy, repeatability, and uniformity, as well as power and weight.
KEYWORDS: Optical spheres, Sensors, Calibration, Long wavelength infrared, Error analysis, Space operations, Thermal modeling, Tolerancing, Temperature metrology, Data modeling
Emissive reference spheres will be used to calibrate the orbiting SPIRIT III LWIR sensor with respect to point sources. Such spheres can be made to approximate very closely the spectrum of an ideal blackbody. However, the temperature of the sphere must be modelled to an accuracy of 1.6K or better during the calibration measurements in spite of various sources of uncertainty over the sphere's near-Earth orbit. Observations of both standard stars and emissive reference spheres will provide an unprecedented opportunity to compare the results of two entirely distinct calibration methods.
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